Amphibious vehicle

ABSTRACT

A sit-astride amphibious vehicle configuration which supports a high performance envelope both on land as well as in water. The vehicle has a planing hull and four retractable wheels. Handlebars provide for directional control in both modes of operation. Each road wheel is retractable by pivoting through at least 45° so as to maximize ground clearance when in the land mode of operation and to minimize drag at substantial lean angles when in the marine mode of operation. While a jet drive may remain directly connected to the engine at all times, the driven wheels are only connected during land mode via a speed-change transmission. The entire power train is supported by a frame that is separable from the hull which in turn has a detachable top deck portion, whereby such configuration simplifies the construction, repair and servicing of the vehicle.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims priority from Great Britain Application Serial Nos. 0423463.9, 0423470.4, 0423474.6, 0423483.7 and 0423517.2, all filed Oct. 22, 2004.

BACKGROUND OF THE INVENTION

The present invention relates to an amphibious vehicle and in particular, to a sit-astride amphibious vehicle having all-terrain vehicle (ATV) capability on land.

Amphibious vehicles are now well known in the art. However, the present applicant has identified a need for an amphibious vehicle having ATV capability on land and which performs as a high speed personal watercraft on water. A number of prior art proposals have proceeded to prototype. However, such vehicles have opted to either optimise operation in the marine mode or, alternatively, operation in the land mode. The result is an amphibious vehicle having poor performance in one mode of operation or the other.

Sit astride amphibious vehicles have previously been described such as in U.S. Pat. No. 5,690,046 to Grzech. Grzech teaches an amphibious tricycle. The problem identified by the inventor in this document, was to provide his personal watercraft (PWC) with limited on land capability in order that it was at least partially mobile when going ashore. GRZECH is limited in that it is only suitable for travel on well made up roads when operating in a land mode. This is because the vehicle has only three wheels and these are connected to the remainder of the vehicle by a suspension arrangement which is adapted to cope only with smooth road surfaces and does not have the suspension travel suitable for off-road use of the vehicle. Also, GRZECH needs a large heat exchanger which protrudes through the keel of the vehicle to provide the very necessary cooling required to cool the highly overpowered marine combustion engine. This dictates against off-road land use since the heat exchanger could easily be damaged in such a use. GRZECH has opted for a single steerable front wheel since this is easiest to provide in an adaptation of a personal watercraft.

Accordingly, there exists the need for an all terrain high speed amphibian vehicle, more particularly, having at least four retractable wheel assemblies.

SUMMARY OF THE INVENTION

In a first aspect, the present invention provides an amphibious vehicle comprising:

a sit-astride seat,

a planing hull,

at least four wheels, each of which is movable between an extended land mode location and a retracted water mode location, two of the wheels being front steerable wheels, which are, at least in the land mode of the vehicle, connected to handlebars which can be operated by a driver to steer the vehicle,

an engine which in the land mode of the vehicle is connected to at least one of the wheels to drive the wheel, and

marine propulsion means to propel the vehicle on water.

GRZECH only seeks to provide a PWC with a limited on-land function; he has taken an existing PWC and adapted it with few changes. GRZECH teaches that the engine can be connected to the driven wheels through a transmission with one fixed gear ratio.

In a second aspect the present invention provides an amphibious vehicle comprising:

a sit-astride seat

a planing hull,

at least four wheels, each of which is movable between an extended land mode location and a retracted water mode location, at least two of the wheels being front steerable wheels, which are, at least in the land mode of the vehicle, connected to handlebars which can be operated by a driver to steer the vehicle,

an engine which in the land mode of the vehicle is connected to at least one of the wheels to drive the wheel, and

marine propulsion means to propel the vehicle on water.

GRZECH in adapting a PWC has sought to keep the overall dimensions of the vehicle within those of an existing PWC; thus he has chosen rear wheels on a trailing arm suspension with a track width less than the hull beam and also a single retractable front wheel.

In a third aspect the present invention provides an amphibious vehicle comprising:

a sit-astride seat,

a planing hull,

at least four wheels, all of which are movable between an extended land mode location and a retracted water mode location, each being pivoted about an axis running fore and aft along the vehicle when moved between the land mode and the water mode locations thereof, two of the wheels being front steerable wheels, which are, at least in the land mode of the vehicle, connected to handlebars which can be operated by a driver to steer the vehicle,

an engine which in the land mode of the vehicle is connected to at least one of the wheels to drive the wheel, and

marine propulsion means to propel the vehicle on water.

In a fourth aspect the present invention provides an amphibious vehicle comprising:

a sit-astride seat,

a planing hull,

at least four wheels, each of which is movable between an extended land mode location and a retracted water mode location, two of the wheels being front steerable wheels, which are, at least in the land mode of the vehicle, connected to handlebars which can be operated by a driver to steer the vehicle,

an engine which in the land mode of the vehicle is connected to at least one of the wheels to drive the wheel, and

marine propulsion means to propel the vehicle on water, wherein:

the planing hull is formed as a single component and the vehicle has one or more deck components all joined to the hull component along a join line which extends around an entire periphery of the vehicle at a level above a water line of the vehicle in water; and

the or at least one of the deck components forms a majority of an upwardly facing surface of the vehicle and is demountable to allow access to the engine located there beneath.

In a fifth aspect the present invention provides an amphibious vehicle comprising:

a sit-astride seat,

a planing hull,

at least four wheels, each of which is movable between an extended land mode location and a retracted water mode location, two of the wheels being front steerable wheels of the vehicle, which are, at least in the land mode of the vehicle, connected to handlebars which can be operated by a driver to steer the vehicle,

an engine which in the land mode of the vehicle is connected to at least one wheel to drive the wheel, and

marine propulsion means to propel the vehicle on water, wherein:

the engine is mounted on a frame releasably connected to the hull, the vehicle also comprising a transmission connecting the engine to the driven wheel(s), at least part of the transmission also being mounted on the frame.

In a sixth aspect the present invention provides an amphibious vehicle comprising:

a sit-astride seat,

a vehicle body having a planing hull,

at least four wheels, each of which is movable between an extended land mode location and a retracted water mode location, two of the wheels being steerable wheels, which are, at least in the land mode of the vehicle, connected to handlebars which can be operated by a driver to steer the vehicle,

an engine which in the land mode of the vehicle is connected to at least one of the wheels to drive the wheel, and

marine propulsion means to propel the vehicle on water, wherein:

the vehicle body defines a pair of footwell areas spaced apart on both sides of the sit-astride seat with the vehicle body having sill portions positioned laterally outside the footwell areas.

These and other features and advantages of the present invention will become apparent from the following detailed description of preferred embodiments which, taken in conjunction with the accompanying drawings, illustrate by way of example the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view from above of an amphibious vehicle according to the present invention;

FIG. 2 is a perspective view from below of the amphibious vehicle of FIG. 1;

FIG. 3 is a top plan view of the amphibious vehicle of FIG. 1;

FIG. 4 is a bottom plan view of the amphibious vehicle of FIG. 1;

FIG. 5 is a side elevation view of the amphibious vehicle of FIG. 1;

FIG. 6 is a front elevation view of the amphibious vehicle according to FIG. 1;

FIG. 7 is a rear end elevation view of the amphibious vehicle of FIG. 1;

FIG. 8 is a view of the vehicle of FIG. 1 in which the top surface of the vehicle has been made transparent;

FIG. 9 is a perspective view of a steering and suspension assembly of the vehicle;

FIG. 10 is a front elevation view of the steering and suspension assembly of FIG. 9, with the wheels in their lowered land mode operation location;

FIG. 11 is the same front elevation view as FIG. 10, but with the wheels raised in marine mode operation;

FIGS. 12-18 correspond to the views shown in FIGS. 1-7 save that the views shown in FIGS. 13 to 18 show the amphibious vehicle with its wheel assemblies retracted for use in marine mode

FIG. 19 is a perspective view of a rolling chassis of the vehicle; and

FIG. 20 is a diagrammatic view of a transmission of the vehicle.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIGS. 1 and 2, there can be seen an amphibious vehicle 10 having a forward bow end 12 and a rear stem end 14.

The vehicle 10 has four road wheels 50,51,52,53 which are connected to the remainder of the vehicle by a wheel suspension system which includes a wheel retraction mechanism for moving the wheels 50,51,52,53 between a lowered state for road use and a raised state for marine use. The front wheels 50 and 53 are steerable and handlebars 54 enable steering of these wheels. The rear wheels 51,52 are driven to propel the vehicle on land. A jet drive unit 55 (see FIG. 2) provides propulsion in marine use.

The structure of the amphibious vehicle 10 comprises an upper deck section 30 and a lower hull section 40. The upper deck structure 30 is sealed to the lower hull section 40 around a peripheral planar edge which is above the water line when the amphibious vehicle 10 is displaced in water—(as can best be seen in FIGS. 5 and FIGS. 8). The complete upper deck section 30 is detachable from the lower hull section 40 as a single unit; this permits ease of access to internal components of vehicle for servicing, etc.

Air inlet openings 31 provide an entry for cooling air (e.g. fan-assisted) for use by the cooling systems of the amphibious vehicle 10. Air entrained via inlets 31 is eventually exhausted via outlets 32. Between air inlet 31 and air outlet 32, a dorade system is installed to prevent the ingress of water. The dorade system facilitates righting of the vehicle on water by use of a labyrinthine air inlet passage system to prevent the ingress of water should the amphibious vehicle 10 be inverted in use in the marine mode. Sit-astride seats 33 and 34 are provided for a driver and a passenger of the amphibious vehicle 10. A footwell area 35 is provided either side of the sit-astride seats 33, 34, each shrouded by bodywork positioned laterally outside of the footwell area 35 to provide protection. These footwell areas 35 may be provided with means to bail automatically any water shipped in use of the amphibious vehicle 10.

Front and rear wheel arches 36, 37 are provided on either side of the amphibious vehicle 10 so as to contain a retractable wheel assembly which is retracted when the amphibious vehicle 10 is operating in the marine mode. An instrument panel 38 is provided ahead of the steering controls to convey relevant parameters of the amphibious vehicle 10 to the driver. Additionally, rear view mirrors (not shown) may be provided as a visual aid to the driver. Furthermore, navigation lights may also be provided within or on the upper deck structure 30 in accordance with the local legislative requirements.

The upper deck structure 30 forms an integral part of the entire structure of the vehicle. It is a structural component and not merely cladding. Typically it will take the form of a composite structure (e.g. glass fibres or carbon fibres set in resin) although any suitable manufacturing method may be employed. Where localised areas of strength are required in the upper deck structure 30, extra layers or mats of fibres may be laid down during manufacture. The deck 30 will be formed with localised reinforced areas in order to provide a complete force transmitting path extending around the vehicle in a complete circle in a plane orthogonal to a longitudinal axis of the vehicle, in order to provide resistance to torsional loads on the vehicle.

Referring now to FIGS. 2 and 4 the underside of the hull can be seen extending from the front bow section 12 to the rear stem section 14. Starting from the planar interface with the upper deck section 30, there is a relatively shallow section 41 extending around a periphery of the amphibious vehicle 30 and in this section there are provided front and rear wheel arches 42, 43. These areas of the hull provide stability when the amphibious vehicle 10 is operated at high speed in marine mode because they provide enclosed volumes spaced laterally from the centre line of the amphibious vehicle 10. As such, when cornering sharply, for example, an increase in righting force is experienced as the angle of lean increases. The bodywork lateral of the footwell areas could be provided with buoyancy inserts which would provide righting forces spaced from the vehicle centre line when the vehicle corners on water.

Cutouts are provided in the hull on either side of the centre line of the vehicle in the region of the front and rear wheel arches 42, 43 to provide slots through which the retractable wheel assemblies can be protracted and retracted. Suitably profiled covers 44, 45 are provided as part of the wheel assemblies so as to reconstruct the lines of the hull when the wheel assemblies are retracted for use in marine mode.

A lower V section 46 depends from the mid section 41 and is provided with a keel section running from the bow 12 of the amphibious vehicle to approximately halfway along the length of the vehicle. At this point, the keel splits to incorporate a water intake area 49 for a jet drive marine propulsion unit of the amphibious vehicle 10. The design of the hull 40 is critical in determining the performance achieved when the amphibious vehicle 10 is operated in the marine mode.

The present applicants have spent considerable time and effort in the design of the hull 40 which has resulted in a rather surprising shape in that usually expected for a planing water craft. The dead rise angle of the hull is substantially 20.7 degrees along substantially its entire length. This compares with traditional planing hulls which start at the bow section with a very steep dead rise angle and these dead rise angles become more shallow along the length of the hull towards the stern, typically ending at 5 degrees or less of dead rise angle.

Since the seating of the vehicle is arranged longitudinally along the vehicle, the vehicle is narrower than a passenger car. Aligning the engine longitudinally along the vehicle gives a body shape which is narrow in beam and deep. Rather than adopting the flat planing hull common in the prior art, the applicant has adopted a greater dead rise angle for the agile marine handling this provides, accepting that this gives a need for a suspension with a lot of travel to give adequate ground clearance on land. Large wheels also enable off-road usage, although they give problems of packaging. Whereas before vehicles such as that of GRZECH strove to keep the track width of the wheels within the beam of the vehicle, the applicant has realised that better land mode operation can be achieved if the track width of the vehicle is greater than the beam of the hull. The approach adopted by the applicant does mean that wheels must be retracted through a large angle in order to be clear of the vehicle waterline in marine use, but the strategy does provide for a vehicle capable both on land and on water.

The hull 40 is additionally provided with hydrodynamic aids in the form of strakes 47, 48 and the profiled suspension arm covers 44, 45 previously referred to. Even with the small footprint of the hull of the amphibious vehicle 10, the hull design 40 is capable of propelling the amphibious vehicle 10 up onto the plane with little difficulty in fast time periods. Furthermore, on-water performance of the amphibious vehicle 10 is not compromised and adequate ground clearance is available in operating the amphibious vehicle 10 in land mode as an all terrain vehicle.

FIG. 8 illustrates location of the major internal components of the amphibious vehicle 10.

In FIG. 8 there can be seen a prime mover 60 which is a multi-cylinder internal combustion engine. It is connected by a transmission 61 to drive the rear wheels 51,52 during land use of the vehicle and to drive the jet drive unit 55 during marine use. As described in other applications of the applicant the jet drive unit is permanently connected to the engine 60 to be driven thereby at all times, whilst the wheels 51 and 52 are connected to the engine 52 only in their lowered land use positions.

In FIG. 8 it can be seen that the handlebars 54 are connected by a steering column 62 to a steering mechanism 63 for steering the front wheels 50,53 of the vehicle which is described in detail in another application of the applicant. Spring and damper assemblies 64,65,66,67 are provided in-board for the wheels 50,51,52,53. Two wheel retraction hydraulic actuators (which cannot be seen in the Figure) are provided, one for the front wheels 50,53 and one for the rear wheels 51,52 to allow the wheels to be retracted from their lowered positions shown in FIG. 8 to their raised positions. These hydraulic actuators will be powered by hydraulic fluid supplies from a pump (not shown) powered by the engine 60.

The seating in the vehicle is provided substantially above the vehicle powertrain, with the handlebars located roughly halfway along the length of the vehicle, this comparing with traditional PWC designs which locate the handlebars roughly two thirds along the length of the vehicle (measured from the back). This gives a good weight distribution for both marine and land use.

The powertrain components illustrated in FIG. 8, i.e. the engine 60, the transmission 61 are built up on a frame platform which is then connected to the hull; this gives considerable advantage for ease of manufacture. Indeed it is envisaged that a chassis could be constructed with a frame supporting all of the wheel suspension components, the wheel steering mechanism, the wheel retraction mechanism, the engine 60 and the transmission 61. This would considerably aid construction and repair. This is illustrated in FIG. 19 where a rolling chassis 300 of the vehicle can be seen stripped of the surrounding hull and deck sections. In the Figure there can be seen the engine, the transmission 61 as well as the suspension assemblies for the front and rear wheels and radiators 70, 302 of the cooling system of the vehicle, all mounted to a common supporting structure 303.

The radiator 70 can also be seen in FIG. 8 located at the front of the vehicle which will cool the vehicle's engine, at least in land use. The vehicle's engine can also be cooled by a water/water heat exchanger (not shown) in marine use, with water being drawn from beneath the vehicle to cool water used by the engine cooling system.

The transmission 61 comprises an output shaft 71 leading drive from the engine to a gearbox 72 which has two output shafts; a horizontally extending shaft 73 taking drive to the jet drive unit 55 and a vertically extending shaft 74 leading to a continuously variable transmission arrangement, the pulleys 75,76 of which can be seen in FIG. 8 and which is shown schematically in FIG. 20. As shown in FIG. 20, the continuously variable transmission has a vertically extending output shaft 400 which extends downwardly to a differential 401 through which drive is relayed to the rear wheels 51,52. The CVT transmission 61 could be replaced in other embodiments by a conventional automatic gearbox or a manual gearbox.

FIGS. 9 to 11 show the front steering and suspension assemblies of the vehicle. A frame 101 has swingably mounted to it left and right suspensions 103 and 105 as seen from the rear of the vehicle.

Each suspension 103 and 105 comprises an upright member 107 (see FIG. 10) connected to a lower suspension arm 109 and an upper wishbone 111. Wheels 50 and 53 (shown in FIG. 1) are each mounted to a hub 114, which is rotatably carried on upright member 107.

Extending from each of the upright members 107 is a steering arm 117 (see especially FIG. 9) to which is pivotally connected a track rod extension 119 at its outer end 121. The inner end of extension 119 is connected to a track rod 123. The track rod 123 is moved transversely by means of link 125 which is connected to a swingable connection 127 on steering column 129.

An actuator 141 having piston rod 143 acts on one arm of swing arm 137 to pivot the arm, the outer ends of which are connected to piston rods 144 of suspension dampers 145 (see FIGS. 10 and 11), which are surrounded by coil springs 146. The base of each damper 145 is connected at 147 to retraction arm 149 pivotally mounted at 151 to frame 101. On retraction the suspension swings about an axis running fore and aft longitudinally along the vehicle; a torsion tube 151 rotates and the lower suspension arm 109 rotates with it (compare FIGS. 10 and 11).

Whilst above a single internal combustion engine is used to both drive the wheels is land mode operations and also to power the jet drive, separate engines could be provided, one for the road wheels and another for the jet drive could be replaced by a propeller.

While a particular form of the present invention has been illustrated and described, it will also be apparent to those skilled in the art that various modifications can be made without departing from the spirit and scope of the present invention. Accordingly, it is not intended that the invention be limited except by the appended claims. 

1. An amphibious vehicle comprising: a sit-astride seat for a driver of the vehicle, a planing hull, at least four wheels, each of which is movable between an extended land mode location and a retracted water mode location two of the wheels being front steerable wheels, which are, at least in the land mode of the vehicle, connected to handlebars which can be operated by a driver to steer the vehicle, an engine which in the land mode of the vehicle is connected to at least one of the wheels to drive the wheel, and marine propulsion means to propel the vehicle on water.
 2. An amphibious vehicle as claimed in claim 1, wherein: the engine drives the marine propulsion means to propel the vehicle on water.
 3. An amphibious vehicle as claimed in claim 1, comprising additionally: a speed change transmission giving the vehicle in forward motion on land a plurality of different gear ratios between the engine and the driven wheel(s).
 4. An amphibious vehicle as claimed in claim 1, wherein: all of the four wheels are each pivoted about an axis running fore and aft along the vehicle when moved between the land mode and the water mode locations thereof.
 5. An amphibious vehicle as claimed in claim 2, comprising additionally: a speed change transmission giving the vehicle in forward motion on land a plurality of different gear ratios between the engine and the driven wheel(s).
 6. An amphibious vehicle as claimed in claim 2, wherein: all of the four wheels are each pivoted about an axis running fore and aft along the vehicle when moved between the land mode and the water mode locations thereof.
 7. An amphibious vehicle as claimed in claim 3, wherein: all of the four wheels are each pivoted about an axis running fore and aft along the vehicle when moved between the land mode and the water mode locations thereof.
 8. An amphibious vehicle as claimed in claim 5, wherein: all of the four wheels are each pivoted about an axis running fore and aft along the vehicle when moved between the land mode and the water mode locations thereof.
 9. An amphibious vehicle as claimed in claim 1, wherein: the two front steerable wheels in land mode locations thereof are located on opposite sides of the vehicle, spaced apart transversely across the vehicle by a first track width; the two other wheels of the four are not connected to the handlebars to be steered thereby and are located at the rear of the vehicle on opposite sides of the vehicle, spaced apart transversely across the vehicle by a second track width greater than the first track width.
 10. An amphibious vehicle as claimed in claim 1, wherein: a separate spring and damper unit is provided for each wheel.
 11. An amphibious vehicle as claimed in claim 1, wherein: a track rod is mounted to extend transversely across the vehicle; the track rod is mounted by mounting means which allow the track rod to be slid along the axis thereof transversely across the vehicle by rotation of the handlebars; the track rod is pivotally connected at one end to a first front wheel and is pivotally connected at a second end to a second front wheel; and the front wheels are pivotally mounted on supporting suspension arrangements for steering rotation under control of the track rod.
 12. An amphibious vehicle comprising: a sit-astride seat, a planing hull, at least four wheels, all of which are movable between an extended land mode location and a retracted water mode location, two of the wheels being front steerable wheels, which are, at least in the land mode of the vehicle, connected to handlebars which can be operated by a driver to steer the vehicle, an engine which in the land mode of the vehicle is connected to at least one of the wheels to drive the wheel, a speed change transmission giving the vehicle in forward motion on land a plurality of different gear ratios connected between the engine and the driven wheel(s), and a jet connected to the engine so as to be directly driven thereby to propel the vehicle on water.
 13. An amphibious vehicle as claimed in claim 12, wherein all of the four wheels are each pivoted about an axis running fore and aft along the vehicle when moved between the land mode and the water mode locations thereof.
 14. An amphibious vehicle as claimed in claim 12, wherein: the speed change transmission comprises a manually operable gearbox and a manually operable gear selection means are provided operable by a driver of the vehicle to select one of the gear ratios.
 15. An amphibious vehicle as claimed in claim 12, wherein: the speed change transmission comprises an automatic gearbox.
 16. An amphibious vehicle as claimed in claim 12, wherein: the speed change transmission comprises a continuously variable transmission.
 17. An amphibious vehicle comprising: a sit-astride seat for a driver of the vehicle, a planing hull, at least four wheels, all of which are movable between an extended land mode location and a retracted water mode location, each being pivoted about an axis running fore and aft along the vehicle when moved between the land mode and the water mode locations thereof, two of the wheels being front steerable wheels, which are, at least in the land mode of the vehicle, connected to handlebars which can be operated by a driver to steer the vehicle, an engine which in the land mode of the vehicle is connected to at least one of the wheels to drive the wheel, and marine propulsion means to propel the vehicle on water
 18. An amphibious vehicle as claimed in claim 17, wherein: each road wheel is pivoted through 45° or more during retraction.
 19. An amphibious vehicle as claimed in claim 17, wherein: the four wheels comprise a front pair of wheels steerable by the handlebars and spaced apart transversely across the vehicle by a first track width when in land mode and a rear pair of wheels spaced apart transversely across the vehicle by a second track width when in land mode; and the planing hull has a maximum beam width which is less than both the first and second track widths.
 20. An amphibious vehicle as claimed in claim 19, wherein: the first track width is greater than the second track width.
 21. An amphibious vehicle as claimed in claim 19, wherein: a separate spring and damper assembly is provided for each wheel.
 22. An amphibious vehicle comprising: a sit-astride seat, a planing hull, at least four wheels, each of which is movable between an extended land mode location and a retracted water mode location, two of the wheels being front steerable wheels, which are, at least in the land mode of the vehicle, connected to handlebars which can be operated by a driver to steer the vehicle, an engine which in the land mode of the vehicle is connected to at least one of the wheels to drive the wheel, and marine propulsion means to propel the vehicle on water, wherein: the planing hull is formed as a single component and the vehicle has one or more deck components all joined to the hull component along a join line which extends around an entire periphery of the vehicle at a level above a water line of the vehicle in water; and the or at least one of the deck components forms a majority of an upwardly facing surface of the vehicle and is demountable to allow access to the engine located there beneath.
 23. An amphibious vehicle as claimed in claim 22, wherein: the hull and each deck component is formed of a composite of fibres set in resin and localised areas of at least some of the deck components are provided with fibre reinforcement additional to the fibre reinforcement in a remainder of the component(s) to provide a greater structural strength in such areas, whereby reinforced areas provide load paths for transmission of loading of the vehicle and the load paths extend around an entire transversely viewed periphery of the vehicle to resist torsion loads acting to twist a front part of the vehicle relative to a rear part of the vehicle.
 24. An amphibious vehicle comprising: a sit-astride seat, a planing hull, at least four wheels, each of which is movable between an extended land mode location and a retracted water mode location, two of the wheels being front steerable wheels of the vehicle, which are, at least in the land mode of the vehicle, connected to handlebars which can be operated by a driver to steer the vehicle, an engine which in the land mode of the vehicle is connected to at least one wheel to drive the wheel, and marine propulsion means to propel the vehicle on water, wherein: the engine is mounted on a frame releasably connected to the hull, the vehicle also comprising a transmission connecting the engine to the driven wheel(s), at least part of the transmission also being mounted on the frame.
 25. An amphibious vehicle as claimed in claim 24, wherein: a suspension system is provided for the road wheels and the suspension system is also mounted on the frame.
 26. An amphibious vehicle as claimed in claim 25, wherein a steering mechanism is connected between the handlebars at the front steerable wheels and the steering mechanism is also mounted on the frame.
 27. An amphibious vehicle comprising: a sit-astride seat, a vehicle body having a planing hull, at least four wheels, each of which is movable between an extended land mode location and a retracted water mode location, two of the wheels being steerable wheels, which are, at least in the land mode of the vehicle, connected to handlebars which can be operated by a driver to steer the vehicle, an engine which in the land mode of the vehicle is connected to at least one of the wheels to drive the wheel, and marine propulsion means to propel the vehicle on water, wherein: the vehicle body defines a pair of footwell areas spaced apart on both sides of the sit-astride seat with the vehicle body having sill portions positioned laterally outside the footwell areas.
 28. An amphibious vehicle as claimed in claim 27, wherein buoyancy chambers are provided in the sill portions of the vehicle. 